As someone who believes nothing I hear and only half of what I see, my first reaction is the unusual nature and details of this incident; the interior of the aircraft shows that it has no front right seat, yet it has a back seat. This seems to be a very unusual configuration for a four seat single; it's neither in a full pax / parachuting / cargo configuration.
Next, the only story about the event comes from the two survivors. There is absolutely no way that the deceased could have 'surprised' either of the other two occupants for the reasons already given; she would have had no previous experience in forcing open Cessna doors (especially during cruise with the high forces required as already mentioned), so there would have been quite some time while she (a) undid her seatbelt, (b) suddenly lunged forward towards the door, (c) grappled with the door handle and started to push on the door. Startled onlookers in the cabin would likely have had more than enough time for the startle effect to diminish enough in order for them to react to this odd behavior.
Perhaps the event did occur, but not as some would want the world to believe.